The only thing more fun than learning to play second-hand instruments is making your own! Shoebox guitars are a favorite of my little rockers. We originally got this project from a Girl Scout (Brownie) Try-Its book, but I've seen them in a number of other places.
1) Start with an old shoebox or other rectangular cardboard box.
2) Cut a rectangular or oval hole in the front for your strings.
3) Use an empty paper towel roll for the neck. Cut a circular hole at one end of your shoebox just big enough to insert the paper towel roll, and insert it.
4) Cut 6 rubber bands to use for strings. Pull them across the opening on the front of your guitar and staple each end. They should be taut but not stretched too far, or they will break or pull out of the staples.
5) Cover your guitar (except for the opening with the strings) and the neck with colored duct tape and decorate to your satisfaction.
6) Invite your friends over for a jam session!
You can also use pots and pans or oatmeal canisters for drums, and make a tambourine by stapling two paper plates together with dried beans or uncooked rice inside, and you have a rock band!